A 66-year-old man presents to the emergency department with a 3-day history of gradually worsening pain and swelling in his left knee. He denies any injury other than a scrape sustained approximately 1 week ago. He reports no fever, redness or warmth to the knee, additional injuries, or other complaints.

The patient’s vital signs are normal except for a temperature of 99.5 °F.  Physical examination is normal except for mild edema and subtle erythema and warmth around the left knee. An abrasion is identified below the knee but does not appear to be infected. Distal pulses at the ankle joint and foot are intact.  

Differential diagnosis include cellulitis, internal derangement of the knee, gout, and septic joint. The patient’s complete blood count and metabolic panel were both normal. A radiograph of the knee is obtained.

What do these images show? What is the next best step in treatment?

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